1. Field of the Invention
This invention relates to high voltage gas insulated transmission lines and more particularly to gas insulated transmission lines having trapping means disposed to remove contaminating particles within the transmission lines.
2. Description of the Prior Art
Gas insulated transmission lines are generally comprised of an outer housing with a center conductor coaxially mounted within the outer housing. The center conductor is conventionally supported by insulating spacers located at intervals along the axis of the transmission line. The line is pressurized with an inert insulating gas medium. The insulating gas is conventionally SF.sub.6.
It has been found that upon installation of the transmission line, dust or other contaminating conducting particles may be left in the system and become charged in the high electric field developed. The AC voltage creates a high AC electric field gradient which alternately lifts and drops the particles and can result in random longitudinal motion within the transmission line. These particles may eventually receive enough energy from the electric field to cross the gap between the outer housing and the center conductor mounted therein, and this can result in high voltage breakdown and failure of the system.
Many methods have been proposed to eliminate the contaminating particles from the transmission lines with some type of trapping means. These methods normally comprise some type of apparatus that creates a low electric field region within the transmission line that is disposed to catch and trap contaminating particles as they move down the line. Examples of such particle traps are found in the U.S. Pat. No. 3,515,939 issued to J. G. Trump and U.S. Pat. No. 4,034,147 issued to R. M. Clark et al. The contaminating particle trap devices disclosed in these patents and similar designs all work with varying degrees of efficiency and effectiveness. There is an ongoing effort by power system engineers to reduce the size and cost of these systems. Reduction in the size of a gas insulated transmission line results in a much higher electric field for the same voltage. This makes efficient particles traps even more critical as particles become more active and deleterious with increasing fields. Another patent of particular interest is U.S. Pat. No. 3,792,188 issued to J. C. Cronin disclosing a corrugated outer housing with the low field regions in the valleys of the corrugated housing. It has been found that higher electric fields greatly reduces the effectiveness of these systems requiring additional trapping means.